Shard with the highest population by far = Atlantic
Shard with the best pvp currently = Lake Superior
Shard with the best Roleplay currently = Catskills
Shard with the kindest community (in my opinion) = Chesapeake
Shards that are usually the best for EM events = Atlantic, Great Lakes, Origin, Chesapeake, Europa. Sonoma, Napa Valley.
I am from Atlantic, and although the population is not as high as it used to be, it is still by far the most populated shard and is considered the "main shard" of the game. The majority of the buying and selling of items in the game still takes place on Atlantic as well.
Now would be a great time to return to UO, since the new Time of Legends expansion is coming out soon